Optical fibers are used to communicate information signals from an optical driver at a proximal end of the fiber to an optical receiver at a distal end of the fiber. To that end, optical fibers comprise cylindrical waveguides. The possible solutions to Maxwell's equations in with cylindrical boundary conditions, or modes, define the propagation patterns of electromagnetic radiation, such as information signals, within an optical fiber.
In theory, the propagating modes in an optical fiber depend on the geometry of the fiber's core and the wavelength of propagation. The ratio of the radial extent of the core to the optical wavelength defines the number of modes (discrete oscillatory solutions to Maxwell's equations) that propagate in the core. As a result, performance of the fiber depends on its wavelength of operation and the geometry of the core. To control the performance of the fiber, it is important to control the core geometry, and as a quality control measure, determine the resultant performance through measurement. For fibers intended for single-mode operation, the most important quality control parameter is the cutoff wavelength of the fiber. At or above the cutoff wavelength, the optical fiber may function as a single-mode fiber, while below the cutoff wavelength, a plurality of modes may propagate in the optical fiber which functions as a multi-mode fiber decreasing the fiber's effective carrying capacity. For fibers intended for multi-mode operation, an important quality control parameter is the core diameter, which by definition includes the behavior of modes which are marginally propagating and will diminish with increasing wavelength and thus become cut off, affecting the core diameter. In practice, there are no sharp discontinuities in the behavior or the modes of an optical fiber. In other words, there is no discrete wavelength at which an optical fiber switches from one set of propagating modes to a smaller set, for example becoming a single-mode operation to a multi-mode operation fiber. Instead, the optical fiber smoothly transitions over a range of wavelengths from a greater set of modes to a smaller set. Given the importance of the cutoff wavelength and core diameter to the practical use of single-mode optical fiber, the cutoff wavelength is, by agreement, practically defined as the wavelength at which the ratio between the total power and the fundamental mode power of an optical fiber is 0.1 decibels (dB). Similarly, multi-mode fiber core diameter is defined by the radius at which the ratio of the power at that radius to the power at zero radius is 2.5%.
Furthermore, these parameters depend on the length and bends of the optical fiber. To ensure that optical fibers deployed in the real world—wherein optical fibers may vary in length and bend many times—optical fiber standards mandate a standard deployment configuration for testing optical fibers. Such tests ensure that optical fibers are consistently manufactured and meet specified parameters of the optical fiber. Example configurations of standardized test deployments are shown in FIGS. 1A and 1B. In particular, FIG. 1A shows an example configuration 100 which has been defined as the ultimate standard for measuring cutoff wavelength. An optical fiber 110 of length L, typically two meters, is connected to a tunable light source 105 and a broadband detector 107. The optical fiber 110 is manipulated into a circle 115 of radius R, typically 14 centimeters. Such an arrangement ensures that a consistent mechanical stress is imposed upon the optical fiber being tested. The entire configuration 100 is implemented upon a flat table, such that the optical fiber 110 remains in an x-y, or two-dimensional, plane. Note that minor variations in the curvature can be accepted.
FIG. 1B shows an alternative configuration 150 of the standardized test deployment 100. The alternative configuration 150 is equivalent to the configuration 100, however the 360 degrees of the circle 115 with radius R are distributed among three curves 155, 160, and 165 with the same radius of curvature R. The alternative configuration 150 is also implemented upon a flat table, such that the optical fiber 110 remains in a two-dimensional plane. To test an optical fiber 110, the optical fiber 110 of length L is connected to the light source 105 and the detector 107 and manipulated on the flat table such that the optical fiber 110 includes the three bends. Other alternative configurations are possible and have been used by manufacturers successfully; the standard requires that any alternative bending configuration be shown to produce equivalent cutoff wavelengths and have no bends of radius smaller than the reference R=140 mm.
The deployment in FIG. 1A is unusable in a practical environment, as it demands either strict control of the fiber length or the ability to change the location of either the source or the detector. While the deployment in FIB. 1B and other equivalent deployments accommodate length variation and have been in use for decades, in practice their use is vulnerable to error. For example, the manipulation of the optical fiber into the appropriate configuration by a human—or in some cases, a robotic device—may introduce small bends in the optical fiber, referred to as microbends, which impose additional mechanical stress upon the optical fiber. As a result, measurements of characteristic parameters, such as cutoff wavelength and core diameter, may be affected. In order to consistently test optical fibers, systems and methods for testing optical fibers which eliminate or substantially reduce the need for manual manipulation of the optical fiber are desirable.
Note that the discussion above is not meant to admit that any of the recognition of problems or needs or issues is known in the art. Rather, such information has been recognized by the inventors of the subject application.